This invention relates generally to monitoring of light sources and more particularly to the suppression of tracking error in the monitoring of the output intensity of laser source, such as in the case of a laser pump module. However, this invention is equally applicable to any other applications where tracking of the intensity or power output of a laser source is required, albeit a semiconductor laser, a fiber laser or a solid-state laser.
In the employment of pump laser modules, such as 980 nm and 1480 nm pump modules, for optical telecommunication applications, it is necessary to insure that the output intensity of the pump laser is maintained at a desired level. This is currently done by monitoring the output power that is provided out of the pump module pigtail fiber using a monitor device, such as a monitor photo diode (MPD), positioned at the back facet of the laser diode chip in the module package. The pump module typically comprises a laser diode chip with its front facet light output provided from the laser cavity aligned to be optically coupled into a single mode pigtail fiber which fiber terminates externally of the package for splicing to a fiber amplifier or fiber laser or other type of optical application. The optical coupling of the laser diode output has been accomplished by means of a lens that collimates and focuses the output light into the input end of the fiber. Some of the laser diode output light is reflected back from the lens back into the laser cavity, where it is amplified in the laser cavity and exits, in part, out of the back facet to the MPD. Another portion of the output light is scattered and lost within the module case or package. Light reflecting from the lens or other optical element may be detected directly without the light passing thorough the diode waveguide.
A more attractive approach for coupling this light is the use of a pigtail fiber that has a lens formed on its input end such as chisel or wedged shaped lens, as disclosed in U.S. Pat. Nos. 5,940,557 by Harker; 5,455,879 by Modavis et al.; 5,500,911 by Roff, and 5,074,682 by Uno et al.; all of which are incorporated herein by their reference. In particular, if the chisel shaped input end of the pigtail fiber is angled relative to the longitudinal axis of the fiber, further improvements in coupling efficiency can be realized as set forth in U.S. Pat. No. 5,940,557. The angled lens with an anti-reflecting (AR) coating placed on its surface prevents a significant portion of laser diode output light reflected off the input chisel lens from reentering the laser diode chip.
As is well known in the art of laser diodes, the back facet of the pump module laser diode has a high reflecting (HR) coating while the front facet has a low reflecting or anti-reflecting (AR) coating so that most of the laser diode optical power in the laser cavity will emanate from the front facet while being highly reflected at the back facet. However, a HR reflector is not a perfect reflector so that approximately 0.5% to 10% of the laser light will penetrate the HR coating and can be employed with the MPD to track the output power of the laser diode by sensing the back facet light from the laser diode. Another way of checking and monitoring the output power is split off a small portion of the output power, e.g. 0.5% or 1% and feed this small amount to an MPD. Typically, the monitor current is going to be about 0.5 to 1 milliamp of current per milliwatt of power from the laser diode chip back facet impinging on the MPD. It has been traditionally preferred to place the MPD at the back facet of the laser diode to take advantage of the small amount power emanating from the back facet of the diode.
One problem with the MPD detector in the package is that with changes in the ambient temperature within the module package for a given output power from the module, the MPD changes in value with such temperature changes. In use of the pump module, end users desire that, for a given MPD current output, a given optical output power can be derived from the module. However, there is always some variation to be expected with changes in the case temperature, but it is required to be within tolerable limits or range, which is now considered between about xc2x15-10% with a package temperature variation from about 0-75xc2x0 C. In other words, a tracking error of MPD with xc2x18% is presently acceptable but values beyond this range are not generally acceptable to end users. Also, the maximum acceptable tracking error will likely be required to be reduced as end user""s demands for higher accuracy continually increase, imposing further suppression of tracking error by pump module manufacturers. Tracking error herein is defined as the change in module output power with the change in case or package temperature for a fixed MPD current developed from the light output collected from the laser diode back facet by the MPD. We have experienced back facet MPD tracking errors in excess of this range and, therefore, something needs to be done to provide for more accurate tracking of the output power of the module to meet the need of end users.
There are several complicated factors in determining the cause of tracking error but two of the principal causes are described as follows. As the module case temperature changes with operation or with ambient temperature, the inside ambient of the pump module package, where the laser diode chip and MPD are positioned, is set to be at a predetermined operating temperature using a thermoelectric cooler (xe2x80x9cTECxe2x80x9d), which may be any number of different operating temperatures but is typically 25xc2x0 C. This is done so that the operating temperature remains the same so the optical characteristics of module operation do not significantly change with ambient temperature.
However, as the module package temperature changes during operation, the package, and particularly the platform supporting the laser diode and the coupling pigtail fiber input end, will flex or warp ever so slightly causing slight internal misalignment between the lensed fiber input tip or end and the laser front facet. This distance or cavity length between the fiber lens and the laser diode front facet is typically around 10 xcexcm. Compared to the cavity length of the laser diode chip, this is quite small. The typical cavity length of a 98 nm chip is about 1.5 mm and the cavity length of a 1480 chip is about 2 mm.
The relative reflective feedback off the lensed fiber tip and the reflected light off of the external surface of the laser front facet form a Fabry-Perot (F-P) cavity. Thus, there are two such F-P cavities existing in the packagexe2x80x94the laser Fabry-Perot (F-P) primary cavity and the facet-to-lens Fabry-Perot (F-P) secondary cavity wherein reflected light from these component surfaces in the secondary cavity achieves some degree of resonance. As the case temperature changes, the distance between the laser front facet and the fiber lens tip can change by a small amount.
Changes in the length of the secondary F-P cavity arising from changes in the case temperature causes the light in this secondary cavity to go into and out of phase with the phase of the light generated in the laser diode chip, adding to and subtracting from the light emitted from the laser diode. This change in phase does not have much effect on the pump module output power because the light reflected between the front facet of the laser diode and the face of the lensed fiber is relatively small compared to the total light output from the laser diode. However, these changes in phase interference can have a significant effect on the MPD because the feedback going into the laser diode from the secondary F-P cavity is amplified in the laser diode chip and the amplified output is detected by the MPD. Thus, the MPD detects a value that is not truly representative of the output intensity of the laser diode and the value detected by the MPD changes with the phase interference between the primary and secondary cavities even though the output power from the module changes very little, if at all.
Another effect on MPD tracking error is fiber lay or positioning in the package, which, due to changes in the birefringence, can change the effective grating strength in the fiber Bragg grating in the pigtail fiber. Changes in case temperature can cause changes in stress on the fiber, particularly in the module snout. Changes in such stress cause changes in the fiber birefringence, which in turn can cause varied amounts of circular polarization. Light reflected off the fiber grating and fed back into the chip will only amplify in one polarization of the light. Thus, changes in stress in the snout with temperature can cause changes in MPD current.
In view of the foregoing, tracking error can be caused by the flexing of the package platform supporting the laser diode chip and the input end of the pigtail fiber, as well as feedback light entering into the laser diode cavity where it is amplified and detected by the MPD in addition to other light emitted from the laser cavity emitted from the back facet.
Therefore, it is an object of the present invention to overcome the aforementioned problems.
It is a further object to suppress or otherwise reduce tracking error in pump module output power monitoring to acceptable levels.
According to this invention, several solutions are provided for suppressing monitoring tracking error.
One solution for reducing tracking error is to employ a biconic lens, instead of a chisel lens, on the input fiber tip in order to suppress the interference caused by the light reflection feedback from the lens on the fiber tip. Additionally, the results are further improved with the biconic lens being angled a few degrees on the input fiber tip relative to the longitudinal axis of the fiber. The angled biconic lens is then spatially positioned from the laser diode output emission with the axis of the laser cavity and, in addition, may be also angularly disposed relative to the longitudinal axis of the pigtail fiber. This has been shown to reduce the change in MPD output current with fixed laser diode power output. The biconic lens has a continuous curved surface whereas the use of a chisel lens has some locally flat surfaces providing stronger feedback reflection. With the use of a biconic lensed fiber input end, there is less feedback of reflected light back into the laser diode cavity. Also, an AR coating may be added to the lens surface to reduce its reflection.
Another solution is to employ a biconic lens whose center is offset from the center of the fiber core by a few microns rather then employing an angularly disposed biconic lens on the fiber. In the case here, the center of the biconic lens radius in the plane of the laser diode junction is laterally offset from the center axis of the fiber input end. As a result, light reflected off of the end of the offset biconic lens would tend to be reflected at an angle to the axis of the laser cavity and, therefore, not fed back into the laser diode cavity. Such offset reflected light would avoid establishing the F-P secondary cavity that leads to tracking error.
Another solution is employing a chisel lens so that a substantial portion of the reflected light from the chisel lens would not reflect back into the laser diode cavity, especially with the additional compound angle placing the axis of laser diode chip at an angle with respect to the axis of the fiber. Angling a chisel lens with respect to the axis of the optical fiber can reduce tracking error at the monitor, e.g. the MPD by avoiding the formation of a strong secondary F-P cavity between the front facet of the laser diode and the chisel lens.
A further solution for reducing tracking error is to strengthen the relatively low coefficient of thermal expansion platform supporting the laser diode source and the coupling fiber supported on the TEC in the package. Preferred materials for such submounts are those with high thermal conductivity. Such materials include ceramics and AlN. By rendering the platform thicker without exceeding the physical limits of the package, the tendency for flexing movement between the laser diode front facet and the lensed input tip or end of the pigtail fiber will be substantially mitigated. This solution in combination with any of the other solutions described and disclosed herein provides for an enhanced suppression of monitor tracking error.
Another solution for reducing this tracking error is to move the MPD to another location in the package rather than at a position at the back facet of the laser diode chip. One such location is adjacent to the coupling region between the laser diode front facet and the lensed input fiber tip where it can detect light lost from the light output from the laser diode front facet. About 30% of the laser light output is typically lost internally in the package due to light divergence and scattering. By detecting light from the front region of the laser diode chip, the laser chip no longer functions as an amplifier of backward reflected light into the laser cavity which magnifies the effects of small changes in the effective front facet reflectivity or changes in the F-P secondary cavity reflectivity. In one embodiment the MPD is to the side of the coupling region and in another embodiment the MPD is beneath the coupling region. Another location is to the side where the MPD monitors reflections off the lensed fiber end.
A still further solution is to increase the reflectivity strength of a fiber Bragg grating formed in the pigtail fiber for feedback of a portion of the light for wavelength stabilization of the laser diode. If the fiber Bragg grating reflectivity level is significantly greater than the reflectivity level of the front facet experienced by the laser diode, small changes in the effective front facet reflectivity or changes in the secondary cavity will have a diminished effect on the changes in the light level emitted from the back facet of the laser diode. Typically the grating reflectivity level may be anywhere between 0.3% to 3% of the transmitted light in the fiber and, further, may be less than the reflectivity of the laser diode front facet.
By increasing its reflectivity level, for example to 6%, changes in package temperature affecting the secondary cavity length or effects in the amount of reflectivity from the lens fiber tip or front facet back into the laser cavity become insignificant due to the comparatively high amount of feedback light from the grating to stabilize the laser diode operation. The use of two or more gratings can reduce the effects arising from birefringence changes in the snout.
Another solution is to coat the end of the fiber lens so as to be more reflective than the reflectivity level of the output front facet or to coat the diode facet so that its reflectivity at the peak wavelength, such as 980 nm, is significantly higher than that off of the surface of the fiber tipped lens, in either case suppressing the establishment of the a F-P secondary cavity. This is because F-P cavities exhibit stronger characteristics if the opposed reflecting surfaces establishing the cavity have similar reflective levels.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.